The proto-oncoprotein c-Abl is a ubiquitous non-receptor tyrosine kinase that is essential for the viability of mice. c-Abl is localized in the cytoplasm and the nucleus. Studies carried out in the current funding period have placed the nuclear c-Abl in a cell cycle-regulated, DNA damage-induced signaling pathway. The upstream regulators of c-Abl are two tumor suppressor proteins. The retinoblastoma tumor suppressor (RB) inhibits c-Abl in G0G1 nuclei. The Ataxia Telangiectasia Mutated gene product (ATM) can further activate c-Abl in S phase nuclei after DNA is damaged. DNA damage-regulated signaling pathways coordinate a number of cellular processes to prevent the transcription, replication or segregation of damaged DNA. Defects in DNA damage-induced regulatory pathways compromise the integrity of the genome leading to tumor development or inappropriate death. Six specific aims grouped into two parts are proposed to explore the biological role of the ATM/c-Abl signaling pathway. The three aims in Part I are focused on the molecular interaction between ATM and c-Abl. Aim 1 will develop an ATM kinase assay with Abl as substrate to study the regulation of ATM function by DNA damage. Aim 2 is to learn whether ATM phosphorylation of c-Abl at Ser(465) regulates the tyrosine kinase activity of Abl. Additionally, an antibody for the phosphorylated Ser(465)-epitope will be developed to use for the in situ study of this ATM-dependent phosphorylation event. Aim 3 is to test whether other domains, in particular the Abl DNA binding domain, have a role in the regulation of Abl by DNA damage. The three aims of Part II are directed at elucidating the biological function of the nuclear c-Abl tyrosine kinase. Aim 4 is to discern what role c-Abl has in ATM regulated processes. These studies will be conducted at the cellular and organismal levels, through the expression of dominant negative and gain-of-function c-Abl mutants in cells and in mice. Aim 5 will implement strategies to identify c-Abl regulated genes. This avenue is suggested by the findings of this group that DNA damage activates the tyrosine phosphorylation of RNA polymerase II through c-Abl and that c-Abl leads to stimulation of transcription. Aim 6 will implement two strategies to identify other nuclear substrates of c-Abl. Identification of substrates will provide further information on the biological function of the c-Abl tyrosine kinase.